Does having ‘Good Genes’ make you sexy?




  • Natural Selections rule #4:
    • If variation in a heritable trait is associated with variation in reproductive success…
    • those traits accumulate in population


  • Brainstorm: What traits might be associated with mating decisions between males and females in the animal kingdom?


  • Natural selection is way broader than you thought!
    • Darwin proposed models of sexual selection

Sexual selection is one kind of natural selection




  • Selection can occur as an association between phenotype and mating success
    • secondary traits in mating
    • Sexual Dimorphism = males and females look different


  • Introduces choosiness into evolution and reproduction
    • ‘mate choice’


  • Often: male ‘showiness’ in nature
    • benefits versus risks…

Is this why males look and behave differently?


Intrasexual selection is the battleground for mating




  • ‘Choosiness’ leads to competition


  • Occurs among males in many species
    • Direct combat
    • Subtle combat
    • Display (rituals and colors)


  • Drives the evolution of secondary sexual characteristics
    • between males

Direct Combat: Stag beetles (Goyens et al. 2016)


Subtle combat



  • Often post-copulatory sperm competition
    • selection of diverse and elaborate male genitalia


  • Common in insects
    • Piskin and Koskin 2010
  • Common in birds


  • “Many bizarre traits, including male genitalia that explode during copulation, male genitalia that “sing” during copulation, potent seminal products that invade the female’s body cavity and her nervous system to influence her behavior…. Eberhard 2009"

Subtle combat


Male displays and rituals (birds of paradise)


Male displays and rituals (widowbird)


Male displays and rituals (bowerbirds)


Why is there sexual dimorphism?



  • Why be choosy?
    • what are the real rewards for the female?


  • How common is choosiness??


  • Sexual selection results in the evolution of sexually dimorphic traits
    • preferential mating
    • direct fitness benefit
    • indirect fitness benefit


  • Heterosexual conversation….for now

Female guppies prefer males with orange spots….Why?


Female blackbirds prefer males with orange beaks.. Why?


Brainstorm: Why do animals prefer symmetric partners?


Runaway selection


Breaking the binary



  • Sex is not just heterosexual in animals
    • ~2,000 species and counting….
    • dragonfly, dolphin, elephant, orca, turkey…


  • Gender binary doesn’t exist in nature
    • Many species with multiple genders
    • 6 different genders described
    • sexual mimicry


  • Intersexual animals
    • Female -> Male, Male -> Female
    • Female and male at same time
  • So is sexual selection a thing?
    • more complicated than it seems on the surface